Method and apparatus for selectively heating a workpiece subjected to low temperature thermomechanical processing

ABSTRACT

Process and apparatus are provided for selective heating a portion of a workpiece to achieve improved ausforming and isoforming processes when the workpiece is plastically deformed by mechanical working above the M s  temperature. The process and apparatus includes a preheating step where a larger portion of the workpiece is preheated to less than the austenitic critical temperature while the final heating step conducted at a temperature higher than the austenitic critical temperature heats a smaller portion of the workpiece which is subsequently subjected to plastic deformation.

This is a division, of now U.S. Pat. No. 4,744,836 application Ser. No.006,142 filed Jan. 23, 1987 which in turn is a continuation-in-part ofapplication Ser. No. 899,323, filed Aug. 28, 1986, now U.S. Pat. No.4,715,707, which in turn was a division of application Ser. No. 752,550,filed July 8, 1985, now U.S. Pat. No. 4,637,844.

BACKGROUND

This invention is particularly applicable to an apparatus and a methodfor inductively heating a predetermined portion of a ferrous workpieceand thereafter mechanically deforming the workpiece shape of the heatedportion to finish-like tolerances while imparting desired physicalproperties to such workpiece in an interrupted quench process and willbe described with particular reference thereto: however, the inventionhas broader applications and may be used with any apparatus or processwhich is capable of selectively heating, in a precisely controlledmanner, a predetermined portion of a metal workpiece so as to permitmechanical shaping of the heated portion of the workpiece at apredetermined temperature to impart desired physical properties to suchworkpiece portion in subsequent surface heat treatments thereof.

The invention will be described with reference to a ferrous workpieceand particularly with respect to plain carbon type steels and alloyedsteels which have already been subjected to a case hardened heat treatprocess. In accordance with broader aspects of the invention however,any metal, ferrous or non-ferrous, which is allotropic or polymorphicmay have physical characteristics which can be modified by the lowtemperature thermomechanical treatment used after selective contourheating of the workpiece.

Isothermal time, temperature transformation curves (hereinafter"isothermal transformation curves") for plain carbon and alloy steelshave long been the foundation for heat treating processes for suchsteels to control their physical properties as well as workpiecedistortion, dimensional variation, quench cracking, etc. When theworkpiece is a gear, cam, bearing, shaft or the like which is subjectedto high contact stresses with other parts, an especially hard, tough,smooth surface must be formed on the case and generally, machinefinished within close tolerances. Apart from those heat treat operationswhich change the chemical composition of the case (such as nitriding),such workpieces are generally produced by infusing or dispersing carboninto the case (at least for low carbon steels) in heat treat operationsconventionally known as carburizing, and carbo-nitriding which arefollowed by cooling the workpiece from an elevated temperature above theA₃ -A_(cm) temperature to some final temperature at a rate determined bythe isothermal transformation curve to achieve certain physicalproperties. As used hereafter, the phrase "austenitic criticaltemperature" or "austenitic temperature" will mean that stabletemperature whereat austenite or gamma iron will exist as aface-centered cubic structure which for iron and carbon irons is shownas the A₃ -A_(cm) transformation temperature curve on the iron-carbonphase diagram.

To minimize quench cracks, control distortion and obtain certain desiredphysical properties related to the grain size of the workpiece, severalinterrupted quench processes, commonly known as austempering andmarquenching have heretofore been used after the workpiece has beencarburized. In conjunction with such interrupted quenches, it has alsobeen known for some time that the tensile strength and ultimate strengthof high hardenability steels can be significantly increased without lossof ductility by mechanically working or deforming the workpiece in thebainite temperature range of the isothermal transformation curveimmediately followed by an oil quench to prevent the formation ofnon-martensitic transformation products. Such treatment is commonlyknown as the Ausform Process. More recently, an enhancement in theAusform Process is disclosed in U.S. Pat. No. 4,373,973 for carburizedgears.

In the '973 patent, a gear steel having certain isothermaltransformation curve characteristics, after carburizing, is reheatedabove the austenitic critical temperature and allowed to cool (at a ratesufficient to pass, without contacting, the "knee" of the isothermaltransformation curve, (i.e., the "critical cooling rate") to atemperature just above the M_(s) temperature (i.e., the temperature atwhich martensite just begins to form and generally shown as theisothermal transformation curve) in a liquid bath whereat the gear teethare swage-rolled (mechanically worked) while the gear remains in themetastable austenitic condition (i.e., the "ausrolling" process).Ausrolling provides the necessary plastic deformation of the workpiecewhich importantly must occur before sufficient time has elapsed (in thebay region of the isothermal transformation curve) to allow any phasetransformation. Upon completion of the swage-rolling, the gear is thenpermitted to air cool or is oil quenched to the martensitic rangefollowed by a conventional tempering process. It was found that in gearswhich were formed from steels having sufficient isothermaltransformation curve characteristics to permit ausrolling, there was afine dispersion of carbides formed during the rolling/swaging operationand that the grain size of the austenite was stabilized to produce avery high dislocation density in the final martensite which was ratheruniformly dispersed throughout the surface. Importantly, the grain sizeand carbon dispersion produced a very smooth surface which could beclosely controlled to finish tolerances. As a result, several finishmachining type operations now required to manufacture close tolerancegears could be eliminated.

Heretofore, the ausrolling process was limited to those steels having asufficiently long metastable austenitic range to permit sufficientswage-rolling of the gear to achieve the desired level of plasticdeformation. The deformation level must be substantial, typically in arange in excess of 60%, to achieve the desired characteristics. When theworkpiece is through heated or even partially through heated by means ofconventional standard atmosphere or vacuum furnaces, to a temperatureabove the austenitic critical temperature, it is difficult to achievesuch level of plastic deformation uniformly throughout the surface ofthe part. That is, if the swage-rolling or mechanical deformation occursbefore the workpiece has attained, throughout its core, the desiredtemperature beneath the knee of the isothermal transformation curve, themetal at the surface of the part where the mechanical working occurswill press against the yielding hotter metal at the core of theworkpiece (since the surface will cool before the core does) and deformin a non-uniform manner. If the swage-rolling operation is delayed untilhomogenization does occur, the time for the plastic deformationoperation is reduced (because the phase transformation will occur at agiven time and higher die forming pressures are required with attendantincreases in energy costs.

The Invention of Prior Application

As noted above, in the conventional interrupted quench processes, asubstantial portion of the workpiece must be heated above the uppercritical temperature and that portion is then cooled at a criticalcooling rate in some liquid bath maintained at or above the M_(s)temperature. The workpiece must remain in the bath until the temperaturethroughout the part is homogenized and this time must be achieved, forany particular steel, within the time limits dictated by that particularsteel's isothermal transformation curve. In accordance with the presentinvention of the prior application, there is disclosed a preheat furnacewhich raises the temperature of the workpiece only to that ofapproximately the M_(s) temperature and an induction coil is used toheat only the contour of the workpiece above the austenitic criticaltemperature to a small case depth. When the workpiece is then quenchedat some temperature below the austenitic critical temperature but abovethe M_(s) temperature, there are several advantages which occur. First,the workpiece which is inductively heated has the same physical andchemical properties in its case as does workpieces treated under theconventional processes. Secondly, the cooling and importantly the rateof cooling of the workpiece is significantly enhanced since the contourof the workpiece is cooled not only by the bath but also by the interiorsurface of the piece. This permits higher critical cooling rates to beachieved, thus making the process more suitable to steels using lessalloying elements than heretofore possible since the knee of theisothermal transformation curve need not be shifted by the addition ofalloying elements. Third, the overall energy requirements are reducedsince only a small portion of the workpiece is heated to the criticaltemperature. Also, since there is less heat inputed into the steel,there is less heat to be dissipated in the bath which makes for a moreprecisely controlled process. Fourth, since the inner portion of theworkpiece is not raised above the upper critical temperature, there isless tendency for the part to experience decarburization, oxidation,grain growth and distortion. In summary, the prior application impartedto the surface of the workpiece physical properties and metallurgicalcharacteristics dictated solely by the isothermal transformation curveand identical to that heretofore achieved when the workpiece wassubjected to through heating, but in a much enhanced and expandedprocess possessing the noted features and capabilities.

THE PRESENT INVENTION

In accordance with the present invention, the applicability of theausforming and ausrolling processes to various metals and industrialapplications and also the use of such processes in "isoforming"applications (defined herein as a thermomechanical process wherein theworkpiece is mechanically deformed, in a plastic state, in whole or inpart, outside of the metastable austenitic range) is materially enhancedby a method which preheats a selected first portion of the workpiece toa temperature at least equal to the approximate M_(s) temperature butless than the austenitic critical temperature for the workpiece; heatinga second portion of the workpiece to a temperature at least equal to theaustenitic critical temperature for the workpiece, the second portionbeing smaller than and contained within the first portion of theworkpiece; forming the desired shape of the workpiece by means of ashaping tool which mechanically deforms the second portion of theworkpiece before the second portion drops below the M_(s) temperatureand then cooling the workpiece below the M_(s) temperature. Theselective heating profile thus generated provides a pinching effect inthat the hot material of the first portion is substantially more ductilethan the temperature of the second portion of the workpiece which hasnot undergone any phase transformation and which acts as a mandrel forthe hotter surface metal. The hot selectively pinched material thusbecomes the only material which is plastically deformed to the desiredlimits during the swage-rolling (or die forming) operation. This methodthus increases or expands the range of ferrous metals which can becommercially subjected to the ausrolling, ausforming and isoformingprocesses in at least two ways. First, because there is less materialwhich is heated above the austenitic critical temperature, there is lessmaterial to cool. This means that more critical cooling rates can beachieved with the present invention. Secondly, the time for theswage-rolling or die forming operation is significantly reduced, thusincreasing the availability of metals and also other manufacturingprocesses which can be improved or enhanced by mechanically forming tofinish or near finish shape the workpiece at elevated temperatures. Itis to be understood that the manufacturing processes under discussionextend to and include not only carburized steel but also other steelsand iron that have been subjected to case hardening operations such ascyaniding, nitriding and carbo-nitriding.

In accordance with another aspect of the invention, the apparatus forfinish forming and heat treating an iron carbide workpiece wouldcomprise preheating means for heating a first portion of said workpieceto a temperature approximately equal to the M_(s) temperature of theworkpiece: final heating means for heating a second portion of theworkpiece to a temperature at least equal to the austenitic criticaltemperature of the workpiece, the second portion of the workpiece beingsmaller than and contained within the first portion of the workpiece:die forming means for forming the second heated portion of the workpiecein its plastic state, and means for cooling the workpiece after it hasbeen shaped.

While in the invention's broader sense, any means for selectivelyheating a portion of the workpiece can be employed such as laser heatingtechniques or vacuum arc heating, or even the various glow dischargetechniques using plasma arcs, it is specifically contemplated thatinduction heating has ideal utilization in commercial applications ofthe present invention. In one form of the invention, the method andapparatus would include a single induction heating coil with a workpiecereceiving opening adapted to surround in closely spaced relationship thesecond portion of the workpiece which is already preheated to atemperature approximately that of the M_(s) temperature temperature(either by means of a separate preheat furnace or by means of the liquidbath), which selectively heats the second portion of the workpiece tothe austenitic critical temperature to achieve the desired plasticdeformation noted above. In this manner, not only will the plasticdeformation occur within limited time spans, but the deformation will beachieved at the desired portion of the workpiece such as the fillet of agear tooth (which is located radially inwardly from the outer edge ofthe gear).

In accordance with another aspect of the subject invention, theinduction heating unit is provided with first and second coils generallymatching but slightly larger in diameter than the second portion of theworkpiece to be heated. The first coil heats the workpiece with a lowradio frequency to provide a substantial heating depth encompassing thesecond portion of the workpiece and insuring that the second portion ofthe workpiece is heated to the M_(s) temperature. The workpiece is thenimmediately moved into the second coil which is operated at high radiofrequency to raise the temperature of the first portion of the workpiecebeyond the austenitic critical temperature. A time delay between theheating steps (and there may be several preheating steps) is utilizedand means for rotating the workpiece within the inductor coils isprovided. Such an induction coil arrangement is disclosed in co-pendingapplication Ser. No. 878,186, filing date June 25, 1986 assigned to thepresent assignee. By using the two coil induction heating arrangementthus described in the present invention, the ability of the process toachieve the desired plastic deformation is enhanced since the core ofthe workpiece may be at a lower temperature than the second portion ofthe workpiece, thus increasing the pinching effect on the second portionof the workpiece to achieve the plastic deformation during mechanicalworking of the workpiece. Additionally, a separate preheat furnace toraise the through temperature of the workpiece to the M_(s) temperatureis not necessary. The dual coil arrangement also permits the firstportion of the heated workpiece to be located away from or removed fromthe circumference or outer edge of the workpiece. In this manner, only adesired portion of the workpiece, i.e., a fillet, may be heat treated inaccordance with the present invention.

While in accordance with the preferred embodiment of the presentinvention, the workpiece would be immediately submersed in the liquidbath after final heating to undergo the die forming process describedabove, in accordance with another aspect of the present invention,believed particularly suited to isoforming processes, the forming toolis positioned immediately adjacent the high frequency inductor coil. Asthe workpiece is progressively heated as it passes through the inductorsand when it emerges from the high frequency coil it is immediatelymechanically worked or deformed and progressively so until the entiredepth of the second portion of the workpiece is thermomechanicallyprocessed. Such progressive heating or scanning process is illustratedin a recently filed continuation-in-part application of application Ser.No. 878,186, filed June 25, 1986 (hereby incorporated by reference)wherein large parts of a large workpiece (i.e., teeth of a large gearhaving a significant depth or axial length) are preheated by lowfrequency induction coils and then finally heated by an induction coilto above the austenitic critical temperature and then quenched as thegear is progressively moved through the inductor coils in a scanningprocess. Control provision may be made to the forming tool or die tosense appropriate levels of resistance and retract the workpiece intothe inductor coils to reheat a portion of the workpiece above the uppercritical temperature level if the forming operation cannot proceedwithin the time limits of the particular isoforming process desired.

It is thus the principal object of the present invention to provide aselective heating process and apparatus, as defined above, to permitausforming, ausrolling and isoforming processes to occur over a largerrange of materials and/or geometries than heretofore possible.

Another object of the present invention is to provide method andapparatus, as defined above, which permits closer finish tolerances tobe maintained in ausrolling, ausforming or isoforming processes thanheretofore possible by utilizing selective heating of the workpiece.

A further object of the subject invention is to reduce the total energyotherwise required in through heating heretofore required in lowtemperature thermomechanical processes under consideration, thusproviding a dimensionally more stable workpiece for mechanical workingand providing greater precision of dimensional control in the finalshape of the workpiece.

Yet another object of the present invention is to provide a moreeconomical system for ausforming, ausrolling or isoforming processesthan heretofore employed by using process and apparatus as definedherein which eliminate the need for separate or additional means forthrough preheating of the workpiece.

Still another obJect of the present invention is the provision of amethod and apparatus, as defined above, which contemplates, preferablyin an isoforming process, the mechanical working or plastic deformationof a workpiece without the need of a liquid bath, thus minimizing thecapital expenditure required for such processes.

Another object of the subject invention is to provide, in the apparatusand process as defined above, a more economical, energy efficient,method and apparatus for inductively heating the workpiece than wouldotherwise be required.

Still a further object of the present invention is the provision of amethod and apparatus, as defined above, which substantially improves theproductivity of an ausforming, ausrolling or isoforming process by meansof selective heating.

Yet another object of the present invention is the provision of a methodand apparatus, as defined above, which utilizes multi-cycle andmulti-frequency processing techniques in induction heating toselectively heat the contour of a workpiece to enhance the surfacequalities obtained in the workpiece by ausforming, ausrolling orisoforming processes, while significantly increasing the surfacehardness of the workpiece.

These and other objects and advantages will become apparent from thefollowing description, taken together with the accompanying drawingsdiscussed in the next section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, schematic side elevational view illustratingthe preferred embodiment of the present invention.

FIG. 2 is an enlarged, partially cross-sectioned, schematic view of theinduction coils of the present invention with the workpiece positionedtherein.

FIG. 3 is an enlarged, partially cross-sectional view similar to FIG. 2,but illustrating the workpiece in a different position.

FIG. 4 is a block diagram setting forth the various steps used inheating the workpiece.

FIG. 5 is an isothermal transformation curve of a workpiece with theausrolling process diagramatically illustrated thereon.

FIGS. 6 and 6a are block diagrams setting forth the variousmanufacturing steps used to produce a finish workpiece in accordancewith the prior art and the present invention, respectively.

FIG. 7 is a schematic electro-mechanical diagram illustrating thecontrols used in the forming tool of the present invention.

FIG. 8 illustrates the deformation of the workpiece resulting from theforming tool.

FIGS. 9 and 10 are isothermal transformation curves of preferred steelsfor the workpiece of the subject invention.

FIGS. 11-20 illustrate isothermal transformation curves with variousthermomechanical processes used in the present invention diagramaticallysuperimposed thereon.

FIG. 21 illustrates a cross-sectional, schematic view of an alternativeembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein the showings are for the purposeof illustrating a preferred embodiment of the invention and not for thepurpose of limiting same, FIG. 1 illustrates apparatus 10 forthermomechanically processing at low temperatures a ferrous workpiece 12moving along a horizontal conveyor path. In the preferred embodiment,workpiece 12 is a hobbed, 31 tooth, 8.5 diametral pitch gear formed froma carburized 9310 steel having an initial gear quality of AGMA 8.However, as noted above, any hypereutectoid, hypoeutectoid, or eutectoidsteel or nodular iron, whether or not case hardened by conventionalprocesses such as carburizing, nitriding, etc. may be used as materialfor the workpiece which may be formed in a wide variety of machineelements such as cams, roller, shafts, splines, bearings, etc.

Apparatus 10 generally comprises an entry work transfer station 16: aliquid bath tank 18 which receives the workpiece 12 from entry worktransfer station 16: a first tank conveyor 20 within tank 18 forconveying the work therein: heating means 22 for removing workpiece 12from tank 18 and heating the workpiece to its austenitic criticaltemperature and then returning workpiece 12 back into tank 18: dieforming means 26 which takes the workpiece 18 from conveyor 20 andmechanically forms the workpiece while it is in a plastic state to afinished shape: a second conveyor 28 within tank 18 for receiving theworkpiece from die forming means 26 and moving workpiece 12 to the exitend of the apparatus: an exit work transfer station 30 similar to entrywork transfer station 16 for removing the workpiece from tank 18, and acooling source 32 adjacent the exit work station 30 for coolingworkpiece 12.

Workpiece 12 enters apparatus 10 by means of entry conveyor 14. In thepreferred embodiment, workpiece 12 has been subjected to a conventionalcarburizing process followed by atmosphere or furnace cooling and is atambient temperature when it enters apparatus 10 via conveyor 14.

The entry work transfer station 16 comprises a platform 34 supported bybearings 35 which roll in a horizontal track 36 suitably secured to aframework not shown. A rack and pinion drive unit 38 moves the platform34 between the illustrated positions shown in FIG. 1. Relative toplatform 34 is a cylindrical sleeve 39 movable in a vertical directionby rack-pinion drive arrangement 40. At the end of sleeve 39 is a collet42. Collet 42 may be any suitable fluidly or mechanically actuateddevice having jaws which fit into the hub of the gear and expandoutwardly by suitable control means to engage and disengage theworkpiece 12 (not shown).

Liquid bath tank 18 is a conventional quenching tank holding anysuitable liquid media which is capable of being maintained at atemperature above the M_(s) temperature of the workpiece. It iscontemplated that a molten salt bath with appropriate silicone basefluids will be utilized. A pump 44 is provided for recirculating thebath medium from a reservoir 45 between the inlet 47 and outlet 48 linesof the tank 18. Conventional bath sensing temperature means, bathagitation mechanisms and heating means for maintaining the liquid mediaof the bath at an appropriate temperature are utilized but not shown.

After workpiece 12 is transferred from inlet conveyor 14 by transfermechanism 16, it is placed in the tank 18 on conveyor 20 and moves tothe heating station 22. It should be noted that while workpiece 12remains in liquid bath tank 18, it is being preheated to a temperaturewhich depends on the temperature of the liquid bath and the time theworkpiece remains in the bath. Reference may now be had to FIGS. 1, 2, 3and 4 for a description of the heating means 22 which comprise a firstpreheater inductor 50, a second, final heat inductor 52 and a supportstructure generally indicated at 54 for rotating and vertically movingworkpiece 12 from a first position within the first preheat inductor 50to a second position within second final heat inductor 52 to a thirdposition on conveyor belt 20 as schematically illustrated in FIG. 1. Thesupport structure 54 schematically includes a cylindrical sleeve 55housing, a shaft 57 containing an expandable collet 58 (similar tocollect actuator 42) for grabbing and releasing workpiece 12. Shaft 57is rotated when either inductor 50, 52 is energized by motor 60. A rackand pinion drive arrangement 61 controlled by a microprocessor (notshown) raises and lowers cylindrical sleeve 55 between the threepositions illustrated.

In accordance with somewhat standard induction heating practice, leads63, 64 of the first preheat inductor 50 are connected across analternating power supply 66, which in practice is a solid state inverterhaving an audio frequency of less than 50 KHz. An appropriate timerfeature of a microprocessor (not shown) utilized in conjunction withalternating power supply 66, energizes and deenergizes first preheatinductor 50 at power and for times needed to perform the presentinvention. Inductor 50 is illustrated as a single turn inductor in thepreferred embodiment, although a two turn inductor may be utilized inplace thereof. Leads 68, 69 of second, final heating inductor 52 areconnected across power supply 71 which in the preferred embodiment, isan oscillator having a high frequency or radio frequency generally over200 KHz and is likewise controlled in timing cycles by a microprocessor(not shown). Coolant liquid, in accordance with standard practice, isdirected through cooling passages 72 of first preheating inductor 50 andpassages 74 of final heating inductor 52, the flow of the coolant intoand out of first and second inductors 50, 52 being illustrated by thehorizontal arrows shown in FIGS. 2 and 3. As shown in FIG. 2, both firstand second inductors 50, 52 are adapted to surround the peripheralsurface of the workpiece, i.e., the crown of the gear teeth in closelyspaced relationship, about 0.05 inches as generally shown as Dimension Ain FIG. 2.

Referring now to FIG. 4, the workpiece 12 is removed from tank liquidbath 18 by support structure 54 and accurately positioned by means ofrack and pinion drive 61 within first, preheater inductor unit 50 asshown in FIG. 1. In this first position, the alternating frequency ofpower supply 66 is directed through leads 63, 64 to first preheaterinductor 50. In the preferred embodiment, the frequency of power supply66 is 3.0 KHz nominal. While workpiece 12 is rotated by motor 60 underthe control of the microprocessor, power supply 66 supplies over 100 kwof energy at 3.0 KHz. In practice, the high power of the initial preheatcycle is 189 kw and the cycle continues for approximately 3.0 seconds.This first high power cycle of relatively low audio frequency current inthe first, single turn, preheat inductor 50 causes heat to flowgenerally along and slightly beyond the root area of the teeth ofworkpiece 12 as shown by line B in FIG. 2. This annular band indicatedby line B or first heated portion of workpiece 12 has a relatively hightemperature while the remainder or the core of the workpiece is at alower temperature. Thereafter, as shown in FIG. 4, the inventioninvolves a delay of approximately ten seconds. During this delay, themodular portions of high temperature within the first heated portion ofthe workpiece, i.e., B, will shrink since energy is dissipated inradiation and conduction. After the delay of 10 seconds, workpiece 12continues to rotate and a second preheat cycle is initiated at 3 KHz or1.4 seconds. The power of this preheat is substantially the same as thepower used in the first preheat cycle. In this instance, the power levelis over about 200 kw and the heat profile of the first annular band isenhanced and maintained at a fairly high temperature, slightly below theaustenitic critical temperature. All preheating occurs while the core ofthe workpiece 12 is at a low temperature, certainly not above that ofthe liquid bath 18 and should a substantial time elapse, conduction andradiation would generally stabilize the temperatures and dissipate theheating profile thus generated. Immediately after the second preheatcycle, the gear is indexed downwardly into the second final heatinginductor 52. This index is a rapid downward index taking less than about0.5 seconds to occur and in practice the shift time is 0.4 seconds. Atthis time, with the workpiece 12 positioned within the second, finalheating inductor 52, a frequency substantially greater than 200 KHz isapplied by power supply 71 through leads 68 and 69 to second inductor52. This frequency, in practice, is 300 KHz at 141 kw for 0.4 seconds.During this high frequency heating, the teeth of workpiece 12 or thesecond portion of workpiece 12 as indicated by "C" in FIG. 2 is heatedabove the austenitic critical temperature. At this point, there is afirst portion of the workpiece heated to a temperature slightly lessthan the austenitic critical temperature and a second portion of theworkpiece smaller than and contained within the first portion which isheated to a temperature beyond the austenitic critical temperature.Workpiece 12 is then rapidly moved into the liquid bath tank 18 for thedie forming operation to occur. As noted, this occurs rapidly but,depending upon the isothermal transformation curve of the material usedand the desired heat treatment, the time before the workpiece is plungedinto the bath may be controlled to insure obtaining the critical coolingrate.

Referring now to FIGS. 1, 7 and 8 for a description of the die formingmeans 26, there is shown in FIG. 1 an entry pivoting structure 80 fortransferring the workpiece into die forming means 26 and a similar exitpivoting structure 81 for transferring workpiece 12 out of die formingmeans 26. Pivoting structure 80 basically comprises an expandable collet83 attached to an arm 84 which is secured to and pivots about a shaft 86which is rotatably driven by a motor 87. Shaft 86 is rotatably supportedin a plate 89 which is secured to hydraulic cylinder 90 in turn fixed toan immovable framework, thus permitting the pivot arm 84 to move in averticle direction, pick up a workpiece within the liquid bath tank 18and move the workpiece into die forming means 26. Similarly, pivotingstructure 81 is similarly constructed so as to remove the workpiece 12from die forming means 26 when the mechanical forming or plasticdeformation shaping is completed.

The die forming means comprise a forming tool die 92 which in thepreferred embodiment is a rolling gear die similar to a hobbing gearwhich is rotatably mounted on a shaft 94 for rotation by motor 96.Rotating shaft 94 is securely mounted to a non-movable frame 97 inliquid bath tank 18 to prevent movement of forming tool die 92.Workpiece 12 is contained within a first C-shaped fixture 98 whichslidingly engages in the vertical direction, by means of a dovetailjoint configuration as at 100, a second C-shape fixture 102 which ismovable in a horizontal direction such as by means of rollers 104 or bya tapered wedge, etc. A horizontally-positioned servo-controlledactuator 106 controls the lateral movement of workpiece 12 into gearforming tool 92 and a load cell 107 interposed between horizontalactuator 106 and second C-shape frame member 102 records the lateralforces between forming tool die 92 and workpiece 12. Similarly, avertically positioned servo-controlled actuator 110 acting against thefirst C-shaped member 98 controls the vertical position of workpiece 12relative to forming tool die 92 with the vertical forces exerted onworkpiece 12 measured by load cell 111. A hydraulic cylinder 114 ismounted on first movable C-shaped fixture 98 and has an appropriate baseportion formed thereon to receive workpiece 12 from the pivotingstructure 80. Workpiece 12 is then moved vertically by hydrauliccylinder 114 within first C-shaped movable fixture 98 to its properposition for the roll-swaging or plastic deformation operation, which isthen controlled solely by actuators 106, 110. Not shown in the schematicillustration of FIG. 1, are timing gears located on a shaft extendingfrom hydraulic cylinder 114 and the shaft 94 of the gear rolling die toinsure synchronization of the workpiece and forming tool die 92.

Referring now to FIGS. 7 and 8, the deformation of workpiece 12 whichoccurs during the swage-rolling action induced by an entry taper in thegear tooth lead of forming tool die 92 as illustrated in FIG. 8 iscontrolled by the closed-loop electrohydraulic circuit schematicallyshown in FIG. 7. An external microprocessor 120 provides a preprogrammedcontrol strategy which senses both position in the horizontal andvertical direction and resistance loads in both directions to controlthe feed of forming tool die 92. As shown in FIG. 7, externalmicroprocessor 120 supervises through a 9,6000 baud serial connectionlinked with a control signal generator 122 (preferably a DaytronicSystems 10 interface unit) to permit multiplexing of the digitallygenerated control signal of microprocessor 120. Horizontal actuator 106which controls the horizontal movement of workpiece 12 includes atransducer (not shown) which consistently generates a horizontalposition feedback to horizontal conditioner and selector 124. At thesame time, horizontal load cell 107 similarly generates a load feedbacksignal to horizontal conditioner and selector 124. Horizontalconditioner selector 124 in turn conditions and selects an appropriatesignal transmitted to a servo-controller 126 for controlling thehorizontal actuator 106 and also an appropriate signal is selected andconditioned and sent to the signal generator 122. The system is underactual position control at all times but during the deformationoperation is also in virtual load control vis-a-vis the command signalsent to servo-controller 126, which is generated by signal generator 122under the control of external microprocessor 120. In this manner, bothhorizontal load and position, control horizontal actuator 106.Similarly, vertical actuator 110 utilizes a position feedback signal tovertical conditioner and selector 128 which also receives a verticalload feedback signal from load cell 111. Vertical conditioner andselector 128 in turn conditions and inputs an appropriate signal tovertical servo-controller 130 and signal generator 122 which in turn maygenerate a command signal to servo-controller 130 for controlling theposition of vertical servo-controlled cylinder 110. Depending on thealogorithm utilized in microprocessor 120, the vertical and horizontalfeeds may be either independent or dependent on one another.

Referring again to FIG. 1, following the deformation operation onworkpiece 12 by gear rolling die 92, the workpiece is transferred fromthe die forming means 26 means of the pivoting structure 81 to conveyor28 and the exit transfer mechanism 30 (which is similar to the entrytransfer mechanism 16) is used to remove workpiece 12 from liquid bath18. The exit transfer mechanism 30 then places workpiece 12 on an exitconveyor 31 where workpiece 12 is cooled below the M_(s) temperature bycoolant source 32. The coolant source could be air, water or some otherliquid directed by nozzle means 33 and a rate sufficient to drop thetemperature of workpiece 12 from the M_(s) temperature to a valueapproximately or approaching the M_(f) temperature. Subsequently,workpiece 12 may be subjected to a conventional tempering process.

METHOD DESCRIPTION

As noted above, the method of the present invention finds particularapplicability to gears which must be manufactured to extremely closetolerances such as would be encountered in aircraft, helicopter, ornuclear submarine applications. FIG. 6 schematically illustrates some ofthe manufacturing techniques which must be utilized to produce gearswhich would be suitable for such applications. The gear teeth aregenerally formed from a gear blank by gear hob and, for the applicationsunder discussion, a shaving cutter operation is required to remove a fewthousandths of an inch of material from the gear teeth. The gear is thensubjected to a normal carburizing process to permit the desired case tobe formed followed by air cooling to obtain the desired dispersion ofthe carbides within the case. The gear is then heated above theaustenitic critical temperature and quenched at a critical cooling rateto produce the desired martensitic structure. Since retained austeniteusually occurs at a higher than acceptable levels, cryogenics or furthercooling may be required to reduce the retained austenite to a level ofapproximately 6 or 7%. The gear then undergoes a normal temperingoperation for stress relieving purposes. The distortion occurring in theteeth following the heat treat operation must now be corrected bygrinding either by the forming of the generating process followed byshot peening and honing to produce the desired surface finish whereuponthe gear is subjected to intensive inspection tests. Such manufacturingprocedure is to be contrasted to that obtained by using the ausrollingprocess employed in the present invention. As shown in FIG. 6a, there isno need to shave the gear after hobbing since the finished tolerancesimposed in the ausforming process will compensate for any lack oftolerance occurring in the hobbing operation. The gear is simplycarburized, reheated above the austenitic critical temperature, followedby an interrupted quench where the ausforming occurs. It has been foundthat the retained austenite is at an acceptable level of 6 to 8% andthat a hob gear with an initial gear quality of AGMA 8 has beentransformed to AGMA 11. Considerable improvements in surface finish havealso resulted eliminating any requirements to either shot peen or honethe gear. Thus, after tempering the gear, only the inspection step needbe accomplished.

As noted above, it is not intended to limit the subject invention tosteels which have been subjected to conventional case hardening heattreatments but to apply the low temperature thermomechanical processingtechniques to a wide class of steel and iron alloys such asthrough-hardened steels, tool steels and austempered ductile iron. Inconsidering steel compositions for such processes, three characteristicsof that steel's isothermal transformation curve must be considered,namely, i) a retarded high temperature transformation time, i.e., thepearlite nose, ii) a sufficiently deep metastable austenitic region topermit sufficient time for plastic deformation of the material, i.e.,the metastable bay and, iii) a preferably 1 temperature to permit anideally low temperature processing. FIGS. 9 and 10 illustrate isothermaltransformation curves of 51B70 steel and 4360 steel, respectively. Bothcurves for both steels are characterized by having a sufficientlyretarded pearlite nose and sufficiently large metastable austenitic bayregions to permit application of the subject invention. However, bothcurves have relatively high M_(s) temperatures in excess of 440° F.Furthermore, there is a tendency when plastically deforming the work, atleast for steels with carburized cases, to increase the M_(s)temperature. However, it is known that the M_(s) temperature can belowered by the addition of carbon and other alloys in accordance withwell established and empirical relationships such as that attributed toAndrews or that of Grange and Stewart. The isothermal transformationcurves shown in FIGS. 9 and 10 also show the temperature levels belowthe M_(s) temperature at which various percentages of martensite areformed.

The isothermal transformation curve shown in FIG. 5 is that of thecarburized steel typically employed in the description of the preferredembodiment above with the ausrolling cooling curve superimposed thereon.The workpiece is generally heated above the austenitic criticaltemperature to a temperature of approximately 1600°-1700°, and is thenquenched at a critical cooling rate sufficient to pass the pearlite kneeof the isothermal transformation curve. The ausrolling process isconducted at a temperature approximately between 400°-600° F., althoughit is contemplated that the plastic deformation could occur at atemperature range of anywhere between 200°-900° F.

The isothermal transformation curves illustrated in FIGS. 11 through 20disclose various ausforming and isoforming processes where the saw toothconfigurations of the cooling curves superimposed on the isothermaltransformation curve indicate mechanical working or plastic deformationof the workpiece in accordance with the teachings of the subjectinvention. Depending upon the desired physical properties of theworkpieces, select thermomechanical processing curves may be utilized.

Referring now to FIG. 21, there is shown an alternative embodimentfalling within the scope of the invention and particularly suited toisoforming processes conducted on workpiece 12 which are carried outwithout the need of a liquid bath. In the embodiment of FIG. 21, acylindrical apparatus such as shown at 54 in FIG. 1 supports workpiece12 in a vertical position and is capable of precise vertical movementsof workpiece 12 from a first preheat inductor 200, to a second finalheat inductor 202 and then to a die forming tool 206 which is rotatablydriven by a motor (not shown) on a movable frame drive in an axialdirection by a servo-controlled actuator (also not shown) to engage inrolling-swaging contact with workpiece 12 for achieving plasticdeformation on the surface thereof. Inductors 200 and 202 are similar tothe inductors disclosed in FIGS. 2 and 3 and are adapted to closelysurround workpiece 12 to produce an air gap 201 in accordance withstandard practice. Inductor 200 is powered by an audio frequency powersupply (not shown), preferably a solid state invertor having an outputbetween 3-10 KHz. As in the preferred embodiment, it is contemplatedthat inductor 200 will be operated for at least two heating cyclesseparated by a time interval to preheat a first portion of workpiece 12as shown by the letters "PH" in FIG. 21 to a temperature slightly belowthe austenitic critical temperature and preferably about 900° F. Theworkpiece is then rapidly moved into the final heat inductor 202 whichis powered by a radio frequency power supply (not shown), preferably aradio frequency oscillator having a frequency about 100-450 KHz andpreferably a frequency above 200 KHz. Second inductor finally heats thesecond portion of the workpiece, shown by letters "FH" in FIG. 21 andwhich is smaller than and contained within the first "PH" portion to atemperature above the austenitic critical temperature. Workpiece 12 isthen moved into contact with die forming tool 206 while the "FH" portionof the workpiece rapidly drops in temperature to below the austeniticcritical temperature.

In the manner thus described, the workpiece 12 can be progressivelypreheated, heated and then plastically deformed by mechanicaldeformation in a somewhat continuous operation. If the time for theplastic deformation becomes too limiting, the second preheat step can bereplaced by another audio frequency inductor placed upstream of firstinductor 200. It is contemplated that a microprocessor controlled,closed-loop electrohydraulic circuit, similar to that disclosed in FIG.7 can be used in the embodiment shown in FIG. 21. The control willalways be forming tool position sensitive but capable of generating anoverriding load induced command signal to the servo-controlled actuatowhich signal (in addition to the position sensing signal) can alsocontrol the power and frequency levels of inductors 200, 202, as well asthe transfer times between the inductors 200, 202 and between theinductor 202 and die forming tool 206. As noted above, should thedissipation of the heat from the second portion, the "PH" portion, dropthe temperature of the "PH" portion to an unacceptable level, the loadon the die forming tool 206 will increase to a level which could triggera retraction of the workpiece 12 into inductors 200 and 202 forreheating, the forming operation then progressing from that point whereit had been initially stopped.

The invention has been described with reference to a preferredembodiment. Obviously, other modifications and alterations will occur toothers upon reading and understanding the specifications. It is myintention to include all such modifications and alterations insofar asthey come within the scope of the present invention.

It is thus the essence of my invention to provide an improved method andapparatus for heating a selected portion of a workpiece which enhancesthe utilization of ausforming, ausrolling and/or isoforming processes.

Having thus described the invention, it is hereby claimed:
 1. Animproved system for isoforming an iron carbide workpiece comprising:(a)preheating means for heating a first band portion of said workpieceadjacent to the contour of said workpieces to a temperatureapproximately equal to the M_(s) temperature of said workpiece while theremaining core portion of said workpiece remains below said M_(s)temperatures; (b) final heating means for heating a second band portionof said workpiece encompassing the contour of said workpiece to atemperature at least equal to the austenitic temperature of saidworkpiece, said second portion smaller than and contained within saidfirst portion of said workpiece; (c) die forming means for forming thesecond heated portion of said workpiece to a finished shape while saidsecond portion of said workpiece is in a plastic state at a temperaturenot less than said M_(s) temperature and supported by said first portionwhile the remaining core portion of said workpiece remains below saidM_(s) temperature; (d) means for controlling movement of said workpieceinto said die forming means to insure that said second portion achievesa critical cooling rate and the temperature thereof becomes less thansaid austenitic temperature; and (e) means for cooling said workpiecebelow said M_(s) temperature after it has been shaped.
 2. The system asdefined in claim 1, wherein said final heating means comprises aninduction heating coil with a workpiece receiving opening; said openingadapted to surround in closely spaced relationship said second portionof said workpiece heated to at least said upper critical temperature;means for moving said workpiece in said opening and means for energizingsaid coil with a high radio frequency.
 3. The system as defined in claim2 wherein said preheating means comprises an induction heating coil witha workpiece receiving opening; said opening adapted to surround inclosely spaced relationship said first portion of said workpiece; meansfor moving said workpiece in said opening and means for energizing saidcoil with a low radio frequency.
 4. The system as defined in claim 3wherein said die forming means further includes a tank containing aliquid and means for maintaining said liquid at a temperature less thanthe austenitic temperature but more than the approximate M_(s)temperature of said workpiece.
 5. The system as defined in claim 4further including means for energizing said first induction coil at alow frequency between 1-50 KHz and means for energizing said secondinduction coil at a high radio frequency greater than about 100 KHz. 6.The system as defined in claim 5 further including means for moving saidworkpiece at a rate producing a delay between said first and second coilof about 1 second.
 7. The system as defined in claim 6 further includingmeans for rotating said workpiece within said first and second coils. 8.The system as defined in claim 4 wherein said die forming means arelocated within said tank.
 9. The system as defined in claim 8 whereinsaid die forming means further includes a die forming tool for shapingsaid second surface and means for sensing pressure from said secondsurface and applying pressure from said tool to said second surface inresponse to said sensed pressure in at least two orthogonal axialdirections.
 10. The system as defined in claim 3 wherein said dieforming means includes a die forming tool in ambient atmosphere adjacentsaid final heat induction coil.